Human cortical expansion involves diversification and specialization of supragranular intratelencephalic-projecting neurons

Abstract

The neocortex is disproportionately expanded in human compared to mouse, both in its total volume relative to subcortical structures and in the proportion occupied by supragranular layers that selectively make connections within the cortex and other telencephalic structures. Single-cell transcriptomic analyses of human and mouse cortex show an increased diversity of glutamatergic neuron types in supragranular cortex in human and pronounced gradients as a function of cortical depth. To probe the functional and anatomical correlates of this transcriptomic diversity, we describe a robust Patch-seq platform using neurosurgically-resected human tissues. We characterize the morphological and physiological properties of five transcriptomically defined human glutamatergic supragranular neuron types. Three of these types have properties that are specialized compared to the more homogeneous properties of transcriptomically defined homologous mouse neuron types. The two remaining supragranular neuron types, located exclusively in deep layer 3, do not have clear mouse homologues in supragranular cortex but are transcriptionally most similar to deep layer mouse intratelencephalic-projecting neuron types. Furthermore, we reveal the transcriptomic types in deep layer 3 that express high levels of non-phosphorylated heavy chain neurofilament protein that label long-range neurons known to be selectively depleted in Alzheimer’s disease. Together, these results demonstrate the power of transcriptomic cell type classification, provide a mechanistic underpinning for increased complexity of cortical function in human cortical evolution, and implicate discrete transcriptomic cell types as selectively vulnerable in disease.

Diversity amongst human cortical pyramidal neurons revealed via their sag currents and frequency preferences

“…Pyramidal cells with somas located in L2 and the upper part of L3 had complex basal dendrites, with apical dendrites often reaching L1. Pyramidal cells located in L3c showed different morphologies, with one cell (cell d) showing simple basal dendrites and another (cell e) showing much more complex basal dendrites, consistent with recent reports on the heterogeneity of pyramidal cells in L3 41 . Lastly, we observed two L5 neurons with very different morphologies: one cell (cell f) displays a simple, "thin-tufted" morphology with apical dendrites terminating at the border of L3 and L4, and another pyramidal cell (cell g) appears "thick-tufted" (similar to those reported in 3 ), with a highly complex basal dendrite and two apical dendrite trunks, with one trunk terminating in upper L3 and the other projecting to lower L3 prior to being truncated due to slicing or optical truncation.…”

Scaled, high fidelity electrophysiological, morphological, and transcriptomic cell characterization

“…The protocol described here, which relies on mouse experiments, was co-developed with an effort to record from neurons from human neurosurgical resections. We found that the critical factors described here generalized well to the human neurons and ultimately led to a study to better understand human excitatory neuron cell types (Berg 2020). Integrating the Patch-seq technique using protocols such as the one described here will be critical in understanding how morphoelectric properties correlate with the tremendous transcriptomic diversity currently being described.…”

“…By default, MIES is packaged with the analysis functions used here, and elsewhere, to map the electrophysiological profile of human and mouse cortical neurons (Berg 2020;Gouwens et al 2019;Gouwens et al 2020;Seeman et al 2018). These stimuli are linked such that a single click generates the entire dataset in < 3 minutes.…”

“…In the Patch-seq technique, the neuron's cytoplasm is collected at the end of the recording, then processed via RNA-seq to identify its gene expression patterns. The technique has been used successfully to characterize both cortical and subcortical neurons (Berg 2020;Scala et al 2020;Hashikawa et al 2020;Gouwens et al 2020).…”

Target cell-specific synaptic dynamics of excitatory to inhibitory neuron connections in supragranular layers of human neocortex

“…Work from several research groups has shown that electrophysiological properties and local connectivity can be studied in acute human neocortical slices derived from surgical resections (Molnar et al, 2008; Jiang et al, 2012; Testa-Silva et al, 2014; Kalmbach et al, 2018; Beaulieu-Laroche et al, 2018; Seeman et al, 2018; Peng et al, 2019). These excised tissues are often distal to epileptic seizure foci or deep brain tumors and in many cases are devoid of overt pathology (Szabadics et al, 2006; Verhoog et al, 2013; Tian et al, 2014; Wang et al, 2015; Beaulieu-Laroche et al, 2018; Berg et al 2020). Human ex vivo neocortical slices can also be cultured for weeks to months (Eugene et al, 2014; Schwarz et al, 2017), and viral transgenesis allows genetic manipulation of cells in brain slices (Andersson et al, 2016; Le Duigou et al, 2018; Ting et al, 2018; Mich et al, 2020; Schwarz et al, 2019).…”